1. Field of the Invention
The present invention relates to vibration isolation devices for machines and, in particular, to elastic mountings for automobile engines or large truck cabins.
2. Background Information
More particularly, the present invention relates to the family of elastic supports realized by the association of elastomer walls and rigid rings, the shape of the assembly being essentially rotationally symmetrical. Such devices are frequently referred to as "elastic bushings", and their deformability is utilized in an essentially radial fashion. One primary purpose of such devices is to react to alternating forces in a direction perpendicular to their principal axis, while supporting a more or less high permanent load. In the principal direction of work (i.e., perpendicular to the principal axis of the device), a damping by hydraulic means is basically associated with the dynamic rigidity presented by such devices to different stresses.
A first type of elastic mounting, the rigidity of which is provided by elastomer pieces which provides integrated hydraulic damping of movements in the same direction as that of the load--the load being applied to the axis of the rotationally symmetrical parts--has recently undergone an evolution by the incorporation therein of a long inertial column of a non-viscous liquid. The resonant action of the column substantially increases the apparent rigidity of the device as soon as the alternating stresses exceed a certain range of frequencies.
An improvement of this principle consists of providing means for "decoupling" the rigidities of the system.
A highly resonant system, by definition, has a coefficient of transmission of accelerations resulting from alternating stresses which are much greater than 1.
"Rigidity decoupling", as that term is used herein, refers to a significant reduction of the apparent rigidity of the inertial device, over the very short strokes which usually characterize high-frequency vibrations. To do this, a short-stroke device is interposed in series with the apparent rigidity, in different forms, and acts in the same way as a low inertia.
The SAE Technical Paper Series No. 86-1412, entitled "Optimum Application for Hydroelastic Engine Mount", dated Sept. 22-25, 1986, presents a mathematical study to which a great number of known systems can be related. Additionally, such devices are disclosed in the following patents: French Patent Nos. 2,467,724 and 2,511,105 (Automobiles Peugeot), German Patent No. 3,019,337 (or European Patent No. 040,290) (Freudenberg).
A great number of improvements have been disclosed by rubber manufacturers, for example, those described in French Patent Nos. 2,599,451 and 2,599,452 (Freudenberg), European Patent No. 0,154,828 (Continental), French Patent Nos. 2,555,273, 2,575,253 (Hutchinson), European Patent Nos. 0,150,824 (Metzeler), 0,115,417 (Avon Industrial Polymers), 0,277,056 and 0,286,527 (Caoutchouc Manufacture et Plastiques, the present applicant) and European Patent No. 0,119,796 or U.S. Pat. No. 4,595,183 (Bridgestone Tire Company Ltd).
Another family of elastic bushings, disclosed, for example, in French Patent No. 2,551,161 (Tokay Rubber), with the improvements disclosed in French Patent Nos. 2,600,737, 2,600,738 and 2,601,098, employs configurations for damping radial elasticity. Similar constructions are disclosed in European Patent Nos. 0,242,254 and 0,248,714 (Hutchinson).
The devices referred to immediately above include two volumes of damping liquid enclosed by elastic lateral walls providing rigidity, and means for limiting the stroke of the radial elastic recall. The improvements relate to the circulation of the liquid between the two chambers by means of a principal passage, in which resonance prevents such circulation of fluid above a critical frequency. The improvements also relate to the incorporation of low-inertia valves which "decouple" this rigidity over a short stroke.
European Patent No. 0,278,801, issued to Caoutchouc Manufacture et Plastiques, discloses the simultaneous fitting of four tubes allowing the formation of chambers, a "resonant column" communication passage therebetween, and thin "decoupling" membranes. These terms may have become accepted usage in the pertinent field of art.
The devices disclosed in the above-referenced documents are usually quite complex in terms of design and manufacture. Many may require the molding of elastomer parts having complicated shapes and may require numerous assembly operations, such as successive fittings of complex parts.
As explained by the mathematical model referred to above, experimentation with designs intended to satisfy various specifications indicate that, once the two rigidities are regulated--one with a blocked volume for regulating high frequencies, and another with a circulating liquid for low regulating frequencies--the apparent damping is then at least partially determined by the dimensional ratios and by the length of the column of damping liquid. Aside from the density of the liquid, it should be noted that the mass of the resonant column connecting the two chambers acts in an inverse ratio of its cross section in relation to the equivalent cross section of a piston which would transfer the same volume of liquid under the action of eccentricity (the "eccentricity" being the relative displacement of the two mounting members of the device under the application of a force).